Focusing a light beam more than thirty focal depths from the aplanatic point with a plano-convex lens

ABSTRACT

A device is described for optically scanning a record carrier with a radiation beam having a high numerical aperture. The radiation beam is focused on the record carrier by means of an objective lens and a plano-convex lens. The plano-convex lens has a gap with the record carrier of several tens of micrometers. It focuses the radiation beam to a point at least 30 focal depths away from an aplanatic point of the plano-convex lens. As a consequence, the lens has a relatively large tolerance for sideways movements.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part of application Ser. No. 09/075,677, filedMay 11, 1998.

FIELD OF THE INVENTION

The invention relates to the field of lens systems for scanning opticalinformation carriers.

BACKGROUND OF THE INVENTION

The invention relates to an optical scanning device for opticallyscanning a record carrier comprising an information layer and atransparent layer, the device comprising an objective lens and apiano-convex lens for converging a radiation beam through thetransparent layer to a focus on the information layer, the plano-convexlens having a convex surface facing the objective lens, and a planarsurface facing the transparent layer.

The amount of information that can be stored on an optical recordcarrier depends inter alia on the size of the radiation spot formed bythe scanning device on the information layer of the record carrier. Theinformation density and hence the amount of stored information can beincreased by decreasing the size of the spot. The spot size can bereduced by increasing the numerical aperture of the radiation beamforming the spot. When using a single objective lens, such an increaseof the numerical aperture is in general accompanied by a decrease of thefree working distance of the lens forming the radiation beam, i.e. thesmallest distance between the record and the lens. At higher numericalapertures, the manufacturing costs of such objective lenses become high,the field of the lens reduces and the dispersion of the material thelens is made of gives increasing problems. The problems may be mitigatedby inserting a plano-convex lens between the objective lens and therecord carrier. The plano-convex lens, sometimes called a slider lens ora solid immersion lens, is arranged at a very small distance above therecord carrier. The convergence of the radiation beam is thendistributed over the objective lens and the plano-convex lens. Anadvantage of the use of the plano-convex lens is that it hardly addsaberrations to the radiation beam.

A scanning device having such a plano-convex lens is known from theEuropean patent application no. 0 727 777. The device comprises anoptical head in which an objective lens and a plano-convex lens convergea radiation beam to a numerical aperture (NA) of 0.84 for scanning therecord carrier. The plano-convex lens is arranged at a small heightabove the record carrier. The lens may be mounted on a slider in slidingcontact with the record carrier or floating on a thin air layer. Thelens has a free working distance of several micrometers. When theoptical head of the device, flying above the surface of the recordcarrier, collides with a dust particle on the surface, the head andrecord carrier will be damaged. The free working distance shouldtherefore be larger than the size of the contamination expected on therecord carrier. A disadvantage of the known device is that themanufacture of the optical head of the device becomes increasinglydifficult at increasing free working distance.

The above citations are hereby incorporated in whole by reference.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a scanning device forscanning a record carrier with a high numerical aperture radiation beam,which can be easily manufactured.

The object is achieved in accordance with the invention by a scanningdevice as described in the opening paragraph, which is characterized inthat the objective lens and plano-convex lens are designed for forming afocus at a distance of more than thirty focal depths of the convergingradiation beam from an aplanatic point of the plano-convex lens.

It has turned out that the difficult manufacture of the known opticalhead is due to the tight tolerance on sideways displacements of theplano-convex lens relative to the position of the objective lens. Thegap between the flat surface of the plano-convex lens and the recordcarrier introduces spherical aberration in the radiation beam which isfocused on the record carrier. The spherical aberration can becompensated in the objective lens. As a result, an aberrated beam entersthe plano-convex lens instead of an unaberrated beam. This aberrationmakes the centring of the objective lens and the known plano-convex lensin a direction perpendicular to the optical axis of the lenses critical.The centring tolerance becomes more tight with increasing free workingdistance. The objective lens and the plano-convex lens are preferablyseparately movable along their optical axes for a proper tracking of therecord carrier. The tight positional tolerance on the known plano-convexlens and the objective lens makes the actuators for these movementsexpensive.

The tight tolerance of the known optical head is a consequence of thedesign of the optical head. In applications where plano-convex lensesare used for forming a high numerical aperture beam, such ashigh-magnification microscope objective lenses and optical heads forscanning high-density optical record carriers, the radiation beam isalways focused in an aplanatic point of the plano-convex lens, becauseonly then the plano-convex lens increases the numerical aperture of thebeam without introducing monochromatic aberrations. See for example thebook “Principles of Optics” by M. Born and E. Wolf, sixth edition,Pergamon Press, 1980, page 253.

The tolerance of the optical head according to the invention issubstantially larger than that of the known optical head because of thedifferent design of the objective lens and the plano-convex lens. Theinventive design does not focus the radiation beam at an aplanatic pointof the plano-convex lens, but at a point which does not coincide withsuch an aplanatic point. The relatively small increase of theaberrations introduced by the plano-convex lens according to theinvention is accompanied by a relatively large increase in the tolerancefor sideways movements of the lens. When the distance between the focuspoint and an aplanatic point is more than thirty focal depths of theradiation beam away from an aplanatic point, the increased tolerancesubstantially simplifies the manufacture of the optical head. A focaldepth is equal to λ/(4(1−cos α)), where λ is the wavelength of theradiation beam and a is the half-angle of the cone of the convergingbeam in air. The focal depth of a converging beam having a half-angle ofα in a medium having a refractive index n is approximately n timeslarger than in air. The numerical aperture NA is equal to (n sin α).When the free working distance is increased to a few tens ofmicrometers, where dust particles can pass between the lens and therecord carrier, the tolerance is relatively large. For gaps larger than50 μm, the distance between the focus point and an aplanatic point ispreferably larger than one hundred focal depths.

To increase the tolerance for thickness variations of the transparentlayer of the record carrier, the refractive index n₁ of the transparentlayer is preferably larger than the refractive index n₂ of theplano-convex lens.

The spherical aberration caused by the gap between the plano-convexlens, having a refractive index lower than n₂, and the transparentlayer, having a refractive index larger than n₂, at least partlycompensate one another. The amount of spherical aberration to beintroduced by the objective lens is then reduced, which in turn furtherincreases the mentioned positional tolerance. The refractive indices arepreferably related through (n₁−1)>1.03 (n₂−1).

The distance between the focus and an aplanatic point depends in generalon the size of the gap between the plano-convex lens and the recordcarrier. The preferred distance measured in micrometers is substantiallyequal to 3Rd_(gap), where R is the radius of the convex surface inmillimeters and d_(gap) the distance between the planar surface and thetransparent layer in micrometers. The distance depends on the actualdesign of the objective lens and the plano-convex lens. The distancewill in general lie within +150% to −50% from the above value, and, ifn₁>n₂, within ±20% of the above value. The focus is preferably arrangedbetween the two aplanatic points of the plano-convex lens.

To obtain a minimum spherical aberration at the location of the focus,the size of the gap, d_(gap), depends preferably on the refractive indexn₂ of the plano-convex lens and n₁ of the transparent substrateaccording to the following relation: d_(gap)/d_(S)=(n₂/n₁ ³)*(n₁ ²−n₂²)/(n₂ ²−1). The value of d_(gap) preferably complies with the relationwithin 40% for values of d_(S) larger than d_(gap), where d_(S) is thethickness of the transparent layer. The value of d_(S) is preferablylarger than the value of d_(gap) in order to obtain refractive indicesof available materials.

The plano-convex lens according to the invention is designed for amagnification different from that of the known plano-convex lens. Theknown lens has a magnification of 1/n₂, where n₂ is the refractive indexof both the lens body and the transparent layer. The known lenstherefore operates in an aplanatic point and complies with the sinecondition. The plano-convex lens according to the invention operatespreferably at a magnification of between 1.1/n₂ ² and 0.99/n₂, and morepreferably within a range between 0.7/n₂ and 0.99/n₂. In this range theamount of spherical aberration compensated in the objective lens issmaller than for an objective lens in the known device which has asimilar size of the gap. This change from the known design makes theplano-convex lens more tolerant to sideways movements at a large freeworking distance. When, additionally, the refractive index n₁ of thetransparent layer is chosen larger than the refractive index n₂ of theplano-convex lens, the tolerance for sideways movements is furtherincreased and the tolerance for thickness variations of the transparentlayer is increased.

A second aspect of the invention relates to an optical scanning devicefor optically scanning a record carrier comprising an information layerand a transparent layer having a refractive index n₁, the devicecomprising an objective lens and a plano-convex lens for converging aradiation beam through the transparent layer to a focus on theinformation layer, the plano-convex lens having a convex surface facingthe objective lens, a planar surface facing the transparent layer, amagnifying power and a refractive index n₂, characterized in that themagnifying power of the plano-convex lens is substantially equal to1/n₁. The specific choice of refractive indices and magnifying powermake the optical head very tolerant for thickness variations of thetransparent layer and a distance between the plano-convex lens and therecord carrier that is relatively independent of the variations in thethickness of the transparent layer. The latter characteristic allows theplano-convex lens to be suspended above the record carrier at a constantheight by e.g. an air bearing, without affecting the compensation of thespherical aberration changes due to the variations in the thickness.

A third aspect of the invention relates to an optical scanning devicefor optically scanning a first and second type of record carrier, thefirst type of record carrier comprising a first information layer and afirst transparent layer having a first thickness, the second type ofrecord carrier comprising a second information layer and a secondtransparent layer having a refractive index n₁ and a second thicknesslarger than the first thickness, the device comprising an objective lensand a plano-convex lens for converging a radiation beam through thefirst or second transparent layer to a focus on the information layer,the plano-convex lens having a convex surface facing the objective lens,a planar surface facing the transparent layer, a magnifying power and arefractive index n₂, characterized in that the magnifying power of theplano-convex lens is substantially equal to 1/n₁. An advantage of thescanning device is that the distance between the plano-convex lens andthe record carrier has substantially the same value for the first andsecond type of record carrier. Hence, the same suspension of theplano-convex lens can be used for both types of record carrier, therebysimplifying the design of the scanning device and improving itstolerance to crashes of the optical head of the scanning device into therecord carrier.

A fourth aspect of the invention relates to an optical scanning devicefor optically scanning a record carrier comprising at least two adjacentinformation layers and a transparent spacer layer having a refractiveindex n₁ arranged between the information layers, the device comprisingan objective lens and a plano-convex lens for converging a radiationbeam to a focus on one of the information layers, the plano-convex lenshaving a convex surface facing the objective lens, a planar surfacefacing the transparent layer, a magnifying power and a refractive indexn₂, characterized in that the magnifying power of the plano-convex lensis substantially equal to 1/n₁. When changing the scanning from oneinformation layer to another one, the axial position of the objectivelens must be changed to move the focus of the radiation beam to thedesired information layer, whereas the position of the plano-convex lensneed not be changed. The distance between the plano-convex lens and theentrance surface of the record carrier can thus be kept at a fixedvalue, thereby simplifying the construction of the optical head of thescanning device.

The value of n₁ is preferably larger than the value of n₂ in the latterthree scanning devices. The refractive indices n₁ and n₂ are preferablyrelated through (n₁−1)>1.03 (n₂−1). The product of the magnifying powerand the refractive index n₁ lies preferably within a range from 0.95 to1.05.

The objects, advantages and features of the invention will be apparentfrom the following more particular description of preferred embodimentsof the invention, as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a scanning device;

FIG. 2 shows an objective lens and a plano-convex lens; and

FIG. 3 shows three different types of record carrier.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a device for scanning an optical record carrier 1. Therecord carrier comprises a transparent layer 2, on one side of which aninformation layer 3 is arranged. The side of the information layerfacing away from the transparent layer is protected from environmentalinfluences by a protection layer 4. The side of the transparent layerfacing the device is called the entrance face 5. The transparent layer 2acts as a substrate for the record carrier by providing mechanicalsupport for the information layer. Alternatively, the transparent layermay have the sole function of protecting the information layer, whilethe mechanical support is provided by a layer on the other side of theinformation layer, for instance by the protection layer 4 or by afurther information layer and transparent layer connected to theinformation layer 3. Information may be stored in the information layer3 of the record carrier in the form of optically detectable marksarranged in substantially parallel, concentric or spiral tracks, notindicated in the Figure. The marks may be in any optically readableform, e.g. in the form of pits, or areas with a reflection coefficientor a direction of magnetization different from their surroundings, or acombination of these forms.

The scanning device comprises a radiation source 6, for example asemi-conductor laser, emitting a diverging radiation beam 7. A beamsplitter 8, for example a semi-transparent plate, reflects the radiationtowards a lens system. The lens system comprises a collimator lens 9, anobjective lens 10 and a plano-convex lens 11. The collimator lens 9changes the diverging radiation beam 7 to a collimated beam 12. Theobjective lens 10, having an optical axis 13, transforms the collimatedradiation beam 12 into a converging beam 14 incident on the lens 11. Thecollimator lens 9 and the objective lens 10 may be combined into asingle lens. The plano-convex lens 11 changes the incident beam 14 intoa converging beam 15, which comes to a focus 16 on the information layer3. The plano-convex lens 11 has a convex surface and a flat surface. Theflat surface faces the transparent layer 2 and forms a gap between thelens and the layer. The piano surface may have a curvature, e.g. foraerodynamic purposes, the curvature being sufficiently small not toaffect significantly the optical performance of the plano-convex lens.Although the objective lens 10 is indicated in the Figure as a singlelens element, it may comprise more elements, and may also comprise ahologram operating in transmission or reflection, or a grating forcoupling radiation out of a waveguide carrying the radiation beam.Radiation of the converging beam 15 reflected by the information layer 3forms a reflected beam 17, which returns on the optical path of theforward converging beam 14. The objective lens 10 and the collimatorlens 9 transform the reflected beam 17 to a converging reflected beam18, and the beam splitter 8 separates the forward and reflected beams bytransmitting at least part of the reflected beam 18 towards a detectionsystem 19. The detection system captures the radiation and converts itinto one or more electrical signals. One of these signals is aninformation signal 20, the value of which represents the informationread from the information layer 3. Another signal is a focus errorsignal 21, the value of which represents the axial difference in heightbetween the focus 16 and the information layer 3. The focus error signalis used as input for a focus servo controller 22, which controls theaxial position of the objective lens 10 and/or the plano-convex lens 11,thereby controlling the axial position of the focus 16 such that itcoincides substantially with the plane of the information layer 3. Thepart of the detection system, including one or more radiation-sensitivedetection elements and an electronic circuit processing the outputsignal of the detection elements, used for generating the focus error iscalled the focus error detection system. The focus servo system forpositioning the lens system comprises the focus error detection system,the focus servo controller and an actuator for moving the lens system.

The gap, i.e. the distance between the planar surface of lens 11 and theentrance surface 5 of the record carrier 1, should be maintainedsubstantially at a nominal value. This can be attained by using apassive air bearing construction carrying lens 11 and designed tomaintain the gap at its nominal value. It is also possible to use anoptically derived error signal which represents the deviation of size ofthe actual gap from its nominal value; a special actuator then keeps theplano-convex lens at its prescribed distance from the transparent layerby using the error signal as an input signal for the actuator servoloop. The actuator of the lens 10 is controlled by the focus errorsignal 21 to keep the focus 16 on the information layer 3.

The spherical aberration which arises when the radiation beam has to befocused through a transparent layer which is thicker than the designthickness of the layer, is compensated for by a focusing action of theobjective lens. The focusing action causes the plano-convex lens toproduce, due to the changing magnification, an amount of sphericalaberration which cancels the aberration produced by the thickertransparent layer. During the focusing action of the objective lens, thegap should be maintained substantially at its nominal value in theabove-mentioned way. The variations in the thickness of the transparentlayer are in general of the order of a few percent of the nominalthickness, e.g. less than 10 μm for a nominal layer thickness of 100 μm.

FIG. 2 shows an enlargement of the objective lens 10 and theplano-convex lens 11. The objective lens 10 may be a mono-asphericalplano-convex lens or a bi-aspherical lens. The objective lens 10 isdesigned in a known way to compensate for the spherical aberrationintroduced by the plano-convex lens 11 and the transparent layer 2,thereby making the radiation beam near the focus 16 nominallysubstantially free from spherical aberration.

Table I shows design parameters of the plano-convex lens 11. Theparameters in the table have the following meaning: d_(gap) is thedistance between the flat surface of the plano-convex lens 11 and theentrance face 5 of the transparent layer 2; R is the radius of theconvex surface of the plano-convex lens; n₂ is the refractive index ofthe lens material; Δs is the distance between the focus point and theaplanatic point corresponding to a magnification of 1/n₂. Δs is measuredin the direction of the aplanatic point corresponding to a magnificationof 1/n₂ ² and in a medium having a refractive index of n₂. B is themagnification of the plano-convex lens in units of 1/n₂. F is theroot-mean-square (RMS) wavefront error close to the focal point at adistance of 30 μm in the field of the lens. D is the RMS wavefront errorfor a sideways displacement of the plano-convex lens of 30 μm. T is theRMS wavefront error when the thickness of the transparent layer 2 is 30μm less than the design thickness.

The wavelength of the radiation is 650 nm and the NA of the convergingbeam 15 is 0.85 in air. The focal depth is equal to 0.343 μm. The designthickness of the transparent layer 2 is identical for the designs 1-7and is equal to 600 μm. The refractive index of the layer is 1.5806,i.e. the refractive index of polycarbonate (PC) at a wavelength of 650nm. The plano-convex lens 11 of the designs 1-3, 5 and 6 is made ofpolycarbonate. The plano-convex lens of the design 4 and 7 is made ofthe glasses BK10 and K5 respectively from the Schott catalog. The axialthickness of the plano-convex lens is defined by the gap thickness, thethickness of the transparent layer, the radius of curvature and themagnification of the plano-convex lens.

TABLE I Design parameters of plano-convex lens d_(gap) Δs B F No. (μm) R(mm) n₂ (μm) (1/n₂) (mλ) D (mλ) T (mλ) 1 50 1.2492 1.5806  0 1.000 24 6157 2 50 1.2492 1.5806  30 0.989 24 55 51 3 50 1.2492 1.5806 295 0.863 3120 47 4 50 1.2492 1.4901 197 0.923 25 16 10 5 25 1.2350 1.5806  0 1.00021 33 29 6 25 1.2350 1.5806 216 0.900 14 15 17 7 25 1.2350 1.5238  920.961 20 13  5

Design no. 1 in the table shows the parameters of a plano-convex lensfor a gap of 50 μm. The refractive index of the lens material is equalto the refractive index of the transparent layer 2, and the focus pointcoincides with the aplanatic point corresponding to a magnification of1/n₂.

Design no. 2 uses the same refractive index as that of PC, but has afocus of the beam at a distance of 30 μm from the aplanatic point. Thisdistance corresponds to 87 focal depths. The RMS wavefront error at 30μm decentring reduces from 61 to 55 mλ. If the scanning device imposes atolerance of 30 μm on the decentring, the 10% reduction of theaccompanying wavefront error can be used advantageously in reducingother tolerances in the often tight optical wavefront error budget ofthe scanning device. Since the wavefront error is to a firstapproximation linear in the decentring, design no. 2 allows a 10% largertolerance on the decentring of the plano-convex lens. The tolerance onthickness variations of the transparent layer has also increased by morethan 10%.

Design no. 3 uses again the refractive index of PC but has a focus at adistance of 295 μm from the aplanatic point. The tolerance on thedecentring has been increased by a factor of three at the cost of aslight reduction of the field of the lens. The tolerance on the layerthickness has also increased.

Design no. 4 uses a refractive index lower than that of the transparentlayer and has a focus 197 μm away from the aplanatic point. The field ofthe lens is about equal to the field of the lens according to the priorart design no. 1. The decentring tolerance has been decreased evenfurther than in design no.3 and the thickness tolerance has increased bymore than a factor of five in comparison with design no. 1.

Design no. 5 shows the parameters of a plano-convex lens for a gap of 25μm. The refractive index of the lens material is equal to the refractiveindex of PC, and the focus point coincides with the aplanatic pointcorresponding to a magnification of 1/n₂.

Design no. 6 uses the refractive index of PC but has a focus at adistance of 216 μm from the aplanatic point. Both the field of the lens,the tolerance on the decentring, as well as the tolerance on the layerthickness have increased by about a factor of two.

Design no. 7 uses a refractive index lower than that of the transparentlayer and has a focus 92 μm away from the aplanatic point. The field isstill larger than the field of design no. 5. The decentring tolerancehas increased even further than in design no. 6. The thickness tolerancehas increased fourfold in comparison with design no. 5. When the opticalbudget of the scanning device for sideways movements is 30 mλ, theplano-convex lens according to design no. 5 has a 27 μm tolerance forsideways movements, whereas the plano-convex lens according to designno. 7 has a 69 μm tolerance.

The tolerance of the optical head for thickness variations of thetransparent layer 2 can be relatively large if the magnifying power B ofthe plano-convex lens 11 is made substantially equal to 1/n₁, i.,e. theinverse of the refractive index of the transparent layer. This can beunderstood as follows. When the distance between lens 10 and lens 11changes, the conjugate distances of lens 11 change. If the imageconjugate of lens 11 changes by an amount of Δ1, the sphericalaberration W in the radiation beam 15 changes by${\Delta \quad W_{1}} = {\frac{1}{8}\left( {1 - B^{2}} \right){NA}^{4}{\Delta l}}$

The spherical aberration introduced by a change Δd₁ in the thickness ofthe transparent layer 2 into the radiation beam 15 is equal to${\Delta \quad W_{2}} = {{- \frac{1}{8}}\frac{1}{n_{1}^{3}}\left( {1 - n_{1}^{2}} \right)\Delta \quad d_{1}\quad {NA}^{4}}$

If the image conjugate of lens 11 changes by Δ1 and the thickness of thetransparent layer by Δd₁, then the distance d₂ between the lens 11 andthe entrance face of the record carrier must change by Δd₂ to keep thefocus of the radiation beam on the information layer, where${\Delta \quad l} = {{\Delta \quad d_{2}} + \frac{\Delta \quad d_{1}}{n_{1}}}$

The spherical aberration introduced by the thickness variation may becompensated by choosing ΔW₁=ΔW₂, resulting in a value of Δ1 equal to${\Delta \quad l} = {{- \frac{1}{n_{1}^{3}}}\frac{\left( {1 - n_{1}^{2}} \right)}{1 - B^{2}}\Delta \quad d_{1}}$

The resulting change Δd_(gap) in the distance d_(gap) between the flatsurface of the plano-convex lens 11 and the entrance face 5 of thetransparent layer 2 is${\Delta \quad d_{gap}} = {{{- \frac{\Delta \quad d_{1}}{n_{1}}}\Delta \quad l} = {{- \frac{\Delta \quad d_{1}}{n_{1}}}\left\lfloor {1 + \frac{\left( {{1/n_{1}^{2}} - 1} \right)}{1 - B^{2}}} \right\rfloor}}$

The term between square brackets can be made zero by taking the value ofB equal to 1/n₁. In that case the change Δd_(gap) in the distanced_(gap) is in this approximation independent of the variations Δd₁ inthe thickness of the transparent layer 2. In other words, the freeworking distance of lens 11 is independent of the thickness of thetransparent layer.

The scanning device is suitable for scanning two different types ofrecord carrier. FIG. 3 shows the first type of record carrier 25,comprising a transparent layer 26 having an entrance face 29 on whichthe radiation of the scanning device enters the record carrier. Therecord carrier 25 also comprises an information layer 27 and aprotective layer 28. The second type of record carrier is the samerecord carrier 1 as shown in FIG. 1, having the transparent layer 2,information layer 3 and protective layer 4. The information layers 3 and27 may have different or similar information densities and theinformation may be stored in different or similar types of marks indifferent or similar formats. The thickness of the transparent layer 26may be zero, making the record carrier a so-called air-incident recordcarrier 30, as shown in FIG. 3, and comprising an information layer 31and a protective layer or substrate 32. In a particular example, thetransparent layer 27 is a 100 μm thickness poly-carbonate foil and thetransparent layer 2 is a 600 μm thickness polycarbonate layer, whereasthe information density of the information layer 27 is higher than thatof information layer 3. The gap between the plano-convex lens 11 and theentrance surface 5 is chosen sufficiently small to realize the requiredhigh numerical aperture of the radiation beam 15 and sufficiently largeto avoid crashes of the plano-convex lens 11 or its holder against therecord carrier.

When the scanning is changed from a record carrier 1 of the first typeto a record carrier 25 of the second type, the plano-convex lens 11 ispositioned at the same height above the entrance face 29 as it had abovethe entrance face 5. The axial position of the objective lens 10 iscontrolled by the focus servo system such that the focus 16 coincideswith the information layer 27. The positioning of both lenses 10 and 11will have changed the distance between them in such a way that thelenses compensate the change in spherical aberration caused by thechange in thickness of the transparent layer. Since the gap between theplano-convex lens 11 and the entrance face of both types of recordcarrier is substantially the same, the dynamic behaviour of the opticalhead will also be very similar for both types of record carrier.

A suitable plano-convex lens 11 for the above scanning device has thefollowing design values. The wavelength of the radiation is 650 nm. Thenumerical aperture for scanning the record carrier 25 having a 100 μmpolycarbonate transparent layer is equal to 0.85. The numerical aperturefor scanning the record carrier 1 having a 600 μm polycarbonatetransparent layer is equal to 0.60. The refractive index ofpolycarbonate at 650 nm wavelength is equal to 1.5806. The lens 11 ismade of the Schott glass FK3 with a refractive index of 1.46. The radiusof curvature of the convex surface of the lens 11 is 1.25 mm, and thethickness of the lens on the optical axis is equal to 1.25 mm. The valueof n₁B for the lens 11 is 0.985. The distance between the plano surfaceand the entrance face 29 of the record carrier 25 is designed to be 100μm. When the scanning is changed to record carrier 1, the distancebetween the piano surface and the entrance face 5 changes to 93 μm, i.e.a change of only 7 μm on a distance of 100 μm.

The scanning device is also suitable for scanning a multi-layer recordcarrier 35 as shown in FIG. 3. The record carrier comprises atransparent layer 36 with an entrance face 37, two information layers 38and 39, separated by a transparent spacer layer 40, and a protectivecover layer 41. The spacer layer may be a foil glued between theinformation layers or a UV-hardened spin-coated layer having a thicknessof 30 μm. The material of the spacer layer may be a polymer such aspoly-carbonate having a refractive index 1.58. The plano-convex lens 11is preferable made according to design no. 4 or 7 of Table I. When thescanning is changed from information layer 38 to information layer 39,the objective lens 10 is moved axially such that the focus 16 isdisplaced from information layer 38 to information layer 39. Thedistance between the plano-convex lens 11 and the entrance face 37 ofthe record carrier does not change, i.e., the free working distancestays approximately the same.

The invention has been disclosed with reference to specific preferredembodiments, to enable those skilled in the art to make and use theinvention, and to describe the best mode contemplated for carrying outthe invention. Those skilled in the art may modify or add to theseembodiments or provide other embodiments without departing from thespirit of the invention. The scope of the invention is not limited tothe embodiments, but lies in each and every novel feature or combinationof features described above and in every novel combination of thesefeatures. Thus, the scope of the invention is only limited by thefollowing claims:

What is claimed is:
 1. An optical scanning device for optically scanninga record carrier comprising an information layer and a transparentlayer, the device comprising an objective lens and a plano-convex lensfor converging a radiation beam through the transparent layer to a focuson the information layer, the plano-convex lens having a convex surfacefacing the objective lens, and a planar surface facing the transparentlayer, the objective lens and plano-convex lens are designed for forminga focus point at a distance of more than thirty focal depths of theconverging radiation beam from an aplanatic point of the plano-convexlens.
 2. The device of claim 1, wherein the transparent layer and theplano-convex lens have refractive indices n₁ and n₂ respectively, thevalue of n₁ being larger than the value of n₂.
 3. The device of claim 2,wherein the refractive indices n₁ and n₂ are related through (n₁−1)>1.03(n₂−1).
 4. The device of claim 1, wherein the distance between the focusand an aplanatic point measured in micrometers is substantially equal to3Rd_(gap), where R is the radius of the convex surface in millimetersand d_(gap) the distance between the planar surface and the transparentlayer in micrometers.
 5. The device of claim 1, wherein the magnifyingpower of the plano-convex lens lies within a range from 1.1/n₂ ² to0.99/n₂, n₂ being the refractive index of the plano-convex lens.
 6. Anoptical scanning device for optically scanning a record carriercomprising an information layer and a transparent layer having arefractive index n₁, the device comprising an objective lens and aplano-convex lens for converging a radiation beam through thetransparent layer to a focus on the information layer, the plano-convexlens having a convex surface facing the objective lens, a planar surfacefacing the transparent layer, a magnifying power and a refractive indexn₂, the value of n₁ is larger than the value of n₂ and the magnifyingpower of the plano-convex lens is substantially equal to 1/n₁.
 7. Thedevice of claim 6, wherein the refractive indices n₁ and n₂ are relatedthrough (n₁−1)>1.03 (n₂−1).
 8. An optical scanning device for opticallyscanning a first and second type of record carrier, the first type ofrecord carrier comprising a first information layer and a firsttransparent layer having a first thickness, the second type of recordcarrier comprising a second information layer and a second transparentlayer having a refractive index n₁ and a second thickness larger thanthe first thickness, the device comprising an objective lens and aplano-convex lens for converging a radiation beam through the first orsecond transparent layer to a focus spot on the information layer, theplano-Convex lens having a convex surface facing the objective lens, aplanar surface facing the transparent layer, a magnifying power and arefractive index n₂, the magnifying power of the plano-convex lens issubstantially equal to 1/n₁.
 9. The device of claim 8, wherein the valueof n₁ is larger than the value of n₂.
 10. The device of claim 9, whereinthe refractive indices n₁ and n₂ are related through (n₁−1)>1.03 (n₂−1).11. An optical scanning device for optically scanning a record carriercomprising at least two adjacent information layers and a transparentspacer layer having a refractive index n₁ arranged between theinformation layers, the device comprising an objective lens and aplano-convex lens for converging a radiation beam to a focus spot on oneof the information layers, the plano-convex lens having a convex surfacefacing the objective lens, a planar surface facing the transparentlayer, a magnifying power and a refractive index n₂, the magnifyingpower of the plano-convex lens is substantially equal to 1/n₁.
 12. Thedevice of claim 11, wherein the value of n₁ is larger than the value ofn₂.
 13. The device of claim 12, wherein the refractive indices n₁ and n₂are related through (n₁−1)>1.03 (n₂−1).
 14. The device of claim 12 inwhich the lenses focus the radiation beam to a focused spot on each ofthe two information layers at different times.